The U.S. Postal Service (USPS) receives and delivers an enormous volume of mail each day through a system that includes regional processing and distribution centers (P&DC's) and local post offices within each region served by a P&DC. See, for example, the diagram presented as FIG. 1 in Allen et al. U.S. Pat. No. 5,422,821. Mail received from postal patrons in each region is sorted at a high level, such as by the first three digits of the zip code at each P&DC, according to a sort scheme that also separates mail that should be sent to one of the local post offices within that region from mail that should be shipped to another P&DC for high level sorting (“outgoing mail”.) Each P&DC thus sends and receives mail from other P&DC's, and also receives for sorting collection mail from within its associated region. Commercial mailers create bulk mailings that qualify for discounted postal rates which bypass some of the channels that collection mail is sent through and is presorted to a greater or lesser degree.
The USPS is committed to achieving delivery times that meet its published goals and seeks to avoid loss of mail in transit. If an error occurs in a sorting process that occurs at a P&DC, such goals can be seriously compromised. The sorting machines used at P&DC's scan mail moving rapidly on a pinch belt conveyor and sort it to one of a large number of bins or pockets. As these pockets become full during the sorting process, a postal worker sweeps the mail from each pocket into a postal tray. Each tray can hold 200-400 mail pieces. Thus, at the end of a sorting run, there are often two or more sets of trays containing mail that will be processed together at the next step, whether that step is a second sort or shipment to another P&DC. These trays are often grouped together on a cart, but trays containing mail from a single pocket in the previous sort may be split among several carts when necessary.
Errors are very costly to the USPS and can occur if some of the mail that should have been transported on to a secondary process is overlooked. If a single tray of mail is misplaced and found later in the P&DC, the mail in the tray will have missed the second step (such as a secondary sort) and have to be processed separately. This will also delay delivery of the misplaced mail to its recipient to a time much later than the stated postal performance goal. At present, however, the USPS must rely on human workers to make certain that mail is transported rapidly and accurately within each P&DC. Occasional errors cannot be avoided, and no automated system has been proposed to eliminate these errors in a manner that is practical considering the circumstances under which postal sorting facilities operate.
Placement of RFID tags on mail pieces in order to identify them has been proposed. See, for example, Sadatoshi et al. U.S. Patent Pub. 20050077353, which allows multiple mail pieces in a tray to be read by RFID. Most RFID readers presently available specify 4 inches between tags because the tag in front relative to the reader shades the one behind it. The system of Sadatoshi et al solves that problem, but it only works if the tray is moved parallel and in close proximity to the reader antenna with mail perpendicular to the plane of movement. This would not be practical for a cart of mail in trays, but could be made to work in a tray management system where the trays are moved down conveyors.
The USPS is implementing a system for tracking mail which they have called an “intelligent mail bar code”, which is a 4-state 65-bar code that can encode 31 digits of information. This will allow mailers to apply a unique tracking identification number to each mailpiece. When the mail arrives at the facility, electronic manifests coupled with a unique tray and/or pallet identifier, such as an RFID tag as described in Pintsov U.S. Pat. No. 6,801,833, can enable a hierarchical tracking of the mail as the trays are removed from the pallet and sent to a sorting system such as a Delivery Bar Code Sorter (DBCS). Pintsov also describes placing an RFID tag on each mail piece, but this has several drawbacks including the cost of the tags and the difficulty doing an RFID tag read on a cart carrying thousands of mail pieces. Even reads of a tray of mail where each piece has an RFID tag are not reliable because the transmission of one tag can “shade” the transmission of adjacent tags. RFID tags cannot be scanned as fast as bar codes and thus it is not practical to scan them as the mail is being transported on a sorting machine conveyor at high speed.
Given that it is not practical to put an RFID tag on each mail piece, identification of the mail in a tray is not possible after the mail has been removed from the original tray and mixed with other mail in a sorting operation. Each time a mail piece is sorted on a sorting machine, the barcode is read and tracking data is available, but it is not practical to keep track of the mail as it is removed from a pocket on the sorter and placed in a mail tray in today's operation. This is because the operator sweeping the system must leave a small amount of mail in the pocket for safety reasons, so it is not known how much of the mail was removed. The operator also typically prints up tray labels in batches and keeps them near the appropriate trays. These labels contain routing information only and cannot be used to uniquely identify trays. To make a unique identifier, the operator would either have to print a unique tag for each tray as the tray is filled, or a means would have to be provided to identify the tray as it is filled. Neither of these methods is practical because the operator is too busy to wait for a tag to be printed and the sorting machines are quite large, so a distributed tray reading system would be very expensive.
The USPS has been tracking mail manually within its facilities for decades, and no workable solution to the problem of tracking mail within a postal sorting center has been found. A practical solution to the foregoing problems with tracking mail within the postal system, i.e. within a single postal sorting plant and as mail is shipped from one facility to another, must require little or no element of human intervention or judgment. Ideally such a process would be fully automated, or nearly so. The present invention provides a solution to this difficult and long-felt problem.